Optical-grade fluorite crystals are grown industrially by directed crystallization by the Bridgeman-Stockbarger method, the basis of which is the moving of crucible containing the melt through a thermal field with specified gradient in high vacuum. A cylindrical container (crucible”) containing the melt is moved from a hot zone to a colder zone. The crystal growth with an oriented seed is used to obtain the crystals with the necessary orientation. From a seed crystal, the crystal is increased gradually in size and occupies all volume of the container that allows growing crystals of given diameter.
One of the main conditions for the growth of quality homogeneous crystals is a constancy of temperature and its gradients in a crystallization zone. The instability of thermal conditions results in occurrence of defects in crystals. Vertical moving (e.g., from the top downward) of the container containing a crystallizing substance from a melt zone into an annealing zone and the smooth or gradual decreasing of temperature under conditions of a constant gradient in the temperature areas is more frequently applied. Thermal screens are used to create the necessary temperature gradients in crystallization zone. The zones are divided from each other with a diaphragm (also called a baffle), (See A. A. Chernov et al., “Sovremennaya kristallographiya” (=Modern crystallography), “Nauka”, Moscow, 1980, V. 3, p. 350.)
Single crystals that have been grown from a melt can have various defects, including defects inherited from a seed crystal. The use of high quality seed crystals is the important condition of obtaining of perfect product crystals. Small diameter seed crystals of the same crystal material, obtained from the crystals that were grown in similar conditions during previous cycles, are usually used. The seed is placed so that a small area of the seed crystal is in the melting zone. The opportunity of obtaining of the perfect crystals from a melt by using the qualitative seed, which has as the growth face a crystal with high symmetry is proved experimentally.
Both radial and axial streams of heat are presented in a growing cylindrical crystal. The radial thermal stream connected with the cooling of an available single crystal, and the axial stream is connected with process of growth. Axial and radial gradients create the residual stress differing as in size and in sign. The initial front of crystallization, which depends on all above mentioned conditions and on other reasons, that accompanies such a delicate and complex physical process as crystallization is formed in a seed zone of melt.
The growth of single crystals by the Bridgeman-Stockbarger method, described in the book entitled “Rost monokristallov” (Growth of Crystals) by R. Lodiz and R. Parker, translated from English into Russian, edited by A. A. Chernov, “Mir”, Moscow, 1974, pp. 181–183), is among the prior art of the present invention. The growth of single crystals in the apparatus, which has two isothermal regions with thermal drop between them is offered that allows annealing a crystal directly after growth without high thermal stress. The two temperature regions should have the minimal heat exchange. They are shared by thermal insulation and the independent temperature regulation in each zone is provided. The heat sink can be carried out from a bottom of the crucible by the use of a special high heat conduction rod, the other end of which is in a cold zone of the furnace or for its limits.
The prior art method of single crystal growth is applicable even for optical fluorite with the use of a seed crystal located inside the crucible with a melt, namely inserted into the central part of crucible at its bottom.
In the prior art, as well as in other sources of the information that are well known to those skilled in the art in the field of the present invention, the specific conditions of the technological process by which one can obtain high quality single crystals of a specific material of the certain sizes, in particular of large size crystals of optical fluorite, are not described. That is, the conditions for making large optical quality fluorite crystals is not known.